FIGS. 13A, 13B and 13C are schematic block diagrams of one example of a conventional electrode pad. An electronic device or optical device fabricated on a semiconductor substrate is usually provided with electrode pads 123 and 124 shown in FIG. 13A.
The electrode pads 123 and 124 are formed on substrates 121 and 122, respectively, and play an important role of supplying from outside an electrical signal for driving the electronic device or optical device and taking to outside an electrical signal amplified or detected by the electronic device or optical device by electrically establishing connection between the electrode pads by a connection wiring 125 such as an metal wire.
If an electrical signal is supplied from outside or an electrical signal is taken out to outside as described above, it is necessary to electrically connect an external electrical device, a wiring track, a connector and the like to the electronic device or optical device by the connecting wire 125. The electrode pad is absolutely necessary for improving workability in the electrical connection or preventing the electronic device or optical device from being impacted in a bonding wire.
Thus, when the electronic device or optical device is designed, the electrode pad should meet the following requirements: (1) it should be capable of being wired reliably in an operation of connection to an external component by bonding wire; (2) it should have a good property of adhesion to the semiconductor substrate 121; (3) it should have a good property of adhesion to an SiO2 film or a low-permittivity insulating material film 126 formed on the semiconductor substrate 121 for preventing short circuit between the substrate and electrode pad when electrode pads are formed on a conductive semiconductor substrate (FIG. 13C); (4) it should be situated such that the function of the electronic device or optical device is not impaired by an impact given in the wiring operation; and (5) the function of the electronic device or optical device should not be limited by the resistance of the electrode pad, a parasitic capacitance or the like. Generally, the electrode pad is designed with consideration given to these requirements (see Non-Patent Document 1 described below).
Non-Patent Document 1: M. N. Khan, et al., “Theoretical prediction and experimental verification of quantum well electroabsorption modulators with bandwidths exceeding 40 GHz”, OFC99, paper ThT4-1/293.